Field of the Invention
The present disclosure relates to an image forming apparatus which forms an image on a sheet.
Description of the Related Art
In an image forming apparatus using electrophotographic technology, a charger charges a photoreceptor, an exposure unit exposes the photoreceptor to form an electrostatic latent image on the photoreceptor, and a developing device develops the electrostatic latent image using toner. The toner image formed in this manner is then transferred to a sheet and fixed on the sheet by a fixing device.
In the image forming method as mentioned, in a case where a toner charge amount in the developing device, a transfer voltage for transferring the toner image or the like is varied in each of the above steps, a density of an output image may vary. Then, a technology which controls an image forming condition for adjusting the density of the output image based on the toner amount supplied to the developing device and the toner amount consumed from the developing device is known (Please see U.S. Pat. No. 8,335,441).
The image forming apparatus disclosed in the U.S. Pat. No. 8,335,441 predicts a variation of the toner charge amount in the developing device based on the toner consumption amount, the toner supply amount and stirring time by a screw in the developing device. Then, based on the predicted variation amount of the charge amount in the developing device, the image forming apparatus controls the image forming condition.
The image forming apparatus disclosed in the U.S. Pat. No. 8,335,441 can correct a variation of image density caused by the variation of the toner charge amount in the developing device. However, it was incapable of correcting the image density variation due to a cause other than the variation of the toner charge amount in the developing device.
A description is provided in a case where an image D1, an image D2, and an image D3 are formed in parallel on one sheet. The image D1 is an image of high density. The image D2 is an image of low density. The image D3 is an image whose density is lower than the image D1 and higher than the image D2. FIG. 8 is a diagram showing a density transition of the image D3 in a conveying direction of the sheet. Each of the images D1, D2, and D3 is formed based on different image signal values. For example, the image signal value for forming the image D1 is 255, the image signal value for forming the image D2 is 10, and the image signal value for forming the image D3 is 100. A result shown in FIG. 8 was obtained by measuring the density of the image D3 using a densitometer by an inventor at one or more positions of different conveying directions.
It is obvious from a lower drawing of FIG. 8 that the image density of an intermediate density portion (image D3) is varied at a boundary between a high density portion (image D1) and a low density portion (image D2). This is the image density variation caused by a transfer current variation caused by a toner adhesion amount in a transfer nip. It is noted that the image forming apparatus as disclosed in the U.S. Pat. No. 8,335,441 cannot treat the density variation which promptly reacts to the variation of the toner adhesion amount in the conveying direction.